1. Field of the Invention
The present invention relates to a signal line structure, and more particularly to a differential signal line structure.
2. Description of the Prior Art
The speed of signal transmission has been improved as the development of technology. At present, the transmission of high speed digital signals is mostly conducted by means of differential signal pair. The high speed digital signals are commonly utilized in the field of Universal Serial Bus 3.0 (USB 3.0), Serial Advanced Technology Attachment (Serial ATA), High-Definition Multimedia Interface (HDMI), Digital Visual Interface (DVI) or radio frequency signal.
In an actual layout of a differential signal line, common mode noise is generated unavoidably due to various reasons and limitations. However, common mode noise causes serious electromagnetic interference (EMI), which should be reduced.
There are many means to reduce the common mode noise generated by the differential signal line, among which nowadays a common mode choke is still commonly utilized in circuits with the working frequency of MHz. Moreover, common mode filters processed by multi-layer substrate and low-temperature co-fire ceramic (LTCC) are also utilized. Even the broadband of an LTCC common mode filter is about 1 GHz; it is still not suitable for the application in high speed/frequency signal circuit. Further, some common mode filters are developed in a direction toward the grounding layer; however, it is not feasible when applied in multi-layer substrates.
Furthermore, electromagnetic band gap (EBG) of a multi-layer substrate is also utilized in common mode filter. Referring to FIGS. 1, 2 and 3, wherein FIG. 1 is an upper view of a common mode filter realized with an EBG of a multi-layer substrate in the prior art; FIG. 2 is a cross-sectional view of a differential signal line structure in the prior art taken along the A-A line of FIG. 1; FIG. 3 is a cross-sectional view of the differential signal line structure in the prior art taken along the B-B line of FIG. 1. In the prior art, a differential signal line 12 is disposed on a layout layer 111 of a substrate 11, a multi-layer substrate 13 is disposed in a filter layer 112 underneath the layout layer 111; the multi-layer substrate 13 is right underneath the differential signal line 12 and is electrically connected to a grounding circuit 15 of a grounding layer 113 through a via 14. As a result, the common mode noise generated by the differential signal line 12 is reduced.
However, it is obvious that the size of the multi-layer substrate 13 is too large to be applied to differential transmission lines, which are arranged in multi-pairs and lately nowadays, regardless of its function of reducing the common mode noise.